Stents are widely used for supporting a lumen structure in a patient's body. For example, a stent may be used to maintain patency of a carotid artery, coronary artery, other blood vessel or other body lumen such as the ureter, urethra, bronchus, esophagus, or other passage. A stent is typically a metal, tubular structure, although polymer stents are known. Stents can be permanent enduring implants, or can be bioabsorbable at least in part. Bioabsorbable stents can be polymeric, bio-polymeric, ceramic, bio-ceramic, metallic, or other materials and stents may elute over time substances such as drugs.
In certain stent designs, the stent is an open-celled tube that is expanded by an inflatable balloon at the deployment site. Another type of stent is of a “self-expanding” type. A self-expanding stent does not use a balloon or other source of force to move from a collapsed state to an expanded state. A self-expanding stent is passed through the body lumen in a collapsed state. At the point of an obstruction, or other deployment site in the body lumen, the stent is expanded to its expanded diameter for its intended purpose. An example of a self-expanding stent is a coil structure that is secured to a stent delivery device under tension in a collapsed state. At the deployment site, the coil is released so that the coil can expand to its enlarged diameter. Coil stents can be manufactured using a variety of methods, such as winding of wire, ribbon, or sheet on a mandrel or by laser cutting from a tube, followed by the appropriate heat treatments. Other types of self expanding stents are closed-cell or open-celled tubes made from a self-expanding material, for example, the Protégé GPS stent from ev3, Inc. of Plymouth, Minn. Cellular tube stents are commonly made by laser cutting of tubes, or cutting patterns into sheets followed by or preceded by welding the sheet into a tube shape, and other methods. The shape, length and other characteristics of a stent are typically chosen based on the location in which the stent will be deployed.
Conventional stents generally are comprised of struts or wires having openings therebetween. During or after stent implantation material can pass from the treatment area through the stent openings and into the lumen of the conduit being treated. This material can separate from the conduit wall, embolize into the lumen, travel downstream and cause problems. For example, atheromatous debris can extrude through stent openings into the lumen of a carotid artery, embolize into the bloodstream, and be carried downstream by blood flow until the embolus becomes lodged in a smaller vessel, causing a stroke. In a similar manner, debris can pass through stent openings in a renal artery, flow distally into the kidney, and embolize, causing impaired renal function, and debris can pass through stent openings in a sapphenous vein graft, flow distally into the myocardium, and embolize, causing impaired heart function.
Some workers in the field have added coverings to stents and thereby substantially occluded the stent openings. However, such covered stents or stent grafts, when compressed into a delivery configuration, tend to be bulky and stiff, thereby making them unsuitable for delivery to small diameter vessels or locations requiring traversal of conduit tortuosity.
Accordingly, a need exists for a stent that is small in profile and flexible when compressed into a delivery configuration and which provides appropriate vessel wall coverage to reduce passage of emboli into the lumen when deployed.